Clergy and supporters put their hands on the shoulder of Chattanooga Deputy Police Chief David Roddy as he speaks during an interfaith memorial service for the Tennessee shooting victims Friday in Chattanooga, Tenn. Muhammad Youssef Abdulazeez opened fire on two military facilities in Chattanooga, killing four Marines and a US Navy sailor.

Doug Strickland/Chattanooga Times Free Press/AP

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There are two ways to catch a potential ‘lone wolf’ terrorist before he or she carries out a violent attack.

One is to gather actionable intelligence that such an attack is being planned – typically through electronic surveillance, often prompted by a tip from a relative or associate – then arresting the alleged plotter before any attack can be carried out. The other is to look for individuals who, for philosophical, religious, political, or personal reasons, may be lured by undercover authorities – often posing as like-minded conspirators – into considering such an attack, then into taking steps in that direction before being arrested.

In the case of Muhammad Youssuf Abdulazeez, who attacked two military facilities in Chattanooga, Tenn., on Thursday, killing four US Marines (and a US Navy sailor who died later) before being shot and killed by police, neither of these scenarios involving known lone wolf terrorists seems to have been part of the violent tragedy. He was “off the radar,” as authorities described him – not having left any trail regarding his intentions, and not part of any official sting operation.

One was a couple of recent blog posts that may sound portentous now but in fact are quite innocuous and did not trigger alarm when they appeared earlier this week.

“Brothers and sisters don’t be fooled by your desires, this life is short and bitter and the opportunity to submit to Allah may pass you by,’’ read one post believed to have been written by Mr. Abdulazeez. “If you make the intention to follow allahs way 100 % and put your desires to the side, allah will guide you to what is right.”

The other clue might have seemed – in retrospect, at least – more serious.

“Counterterrorism investigators are looking closely at a [seven month] trip the Chattanooga shooting suspect took to Jordan in 2014 to determine if he had contact with any extremists or traveled to other countries, according to people familiar with the investigation,” The Wall Street Journal reports. “Law-enforcement and intelligence agencies are now trying to determine if he made connections, or drew inspiration, on that trip to later commit an act of violence once he returned to the U.S., these people said.”

Abdulazeez, a 24-year-old naturalized US citizen of Jordanian descent who was born in Kuwait, had lived in this country with his family – parents and sisters – for years. (Jordanian sources denied that he was a Jordanian citizen, CNN reported, but rather a Palestinian who carried a Jordanian travel document.)

Described by friends, teachers, and coaches as a popular, friendly young man, he graduated from the University of Tennessee at Chattanooga in 2012 with a degree in electrical engineering. According to some reports, he worked briefly at a nuclear power plant.

Abdulazeez may have turned increasingly religious, as evidenced by comments from some who knew him and by his growing a beard. But in April, he was arrested for driving under the influence (arresting officers suspected drugs as well as alcohol) – hardly something that might have been expected of a devout Muslim.

Was he reacting to an upbringing and family situation that might have affected his mental and emotional stability?

The Chattanooga Times Free Press reports that Abdulazeez “grew up in an abusive home.”

“In a divorce complaint filed in Hamilton County Chancery Court, Rasmia Ibrahim Abdulazeez alleged that Abdulazeez's father, her husband, beat and verbally abused his wife in front of his children,” the newspaper reports. “According to the complaint for divorce, the elder Abdulazeez also beat his children, striking them and yelling at them ‘without provocation or justification.’”

The divorce complaint was withdrawn and the lawsuit dismissed in 2009 after Youssuf Abdulazeez agreed to sign a postnuptial agreement and promised to attend counseling, both by himself and with his wife.

Years ago, according to other press reports, the senior Mr. Abdulazeez was investigated for giving money to an organization with possible ties to terrorism. He was placed on a watch list from which he was later removed. No charges were ever filed against him.

While they are treating this week’s deadly shooting as an act of domestic terrorism, officials say their initial investigation has produced no links to any other individual or terrorist group.

Still, as the Monitor’s Warren Richey reported Friday, Islamic State leaders have appealed for attacks to be directed against military personnel and police officers in the US. In addition, they have urged that such attacks take place during the Muslim holy month of Ramadan, which ended Thursday.

According to the US House of Representatives Homeland Security Committee’s most recent “Terror Threat Snapshot,” with the recent attacks in France and against tourists in Tunisia, the Islamic State (also known as ISIS) has now been linked to 47 terrorist plots or attacks against the West, including 11 inside the United States.

“The total number of ISIS arrests in the US has increased five-fold in 2015 with 55 ISIS supporters detained, ten of which have been arrested in the last month,” the report states. “ISIS followers have now been arrested in at least 19 states.”

It may well be that Abdulazeez was inspired by the Islamic State’s encouragement of lone wolf attacks on US targets. But the methods used to head off such plots in the past – plans and aspirations detected through electronic surveillance (including the tracking of social media), or sting operations set up by the FBI and other law enforcement agencies – appear not to have been options in Chattanooga.